Total: "may be months" to stop North Sea gas cloud

Shipping was ordered to come no closer than two miles from the Elgin platform and aircraft no nearer than three miles if they flew lower than 4,000 feet - effectively shutting out helicopters but not affecting airline traffic.

For Scotland's government, environment secretary Richard Loch head said: "Impact on the environment ... is minimal."

Scientists said the gas, flammable methane but containing poisonous hydrogen sulphide - familiar from the smell of rotten eggs - should disperse in the atmosphere. But it poses a risk to anyone close to the source, making capping the well complex.

Poison in the gas could also threaten fish and other marine life nearby, although the rate at which it dissipates in air and water meant it was not a significant threat to people on land.

Greenpeace used the accident to criticize new incentives offered by the British government for deep water drilling west of Shetland and called North Sea energy production dangerous to those working there and damaging to the environment.

The campaign group's Vicky Wyatt said the Total leak was "a reminder of the dangers that drilling for oil and gas pose to the lives of those working on rigs, and the huge damage that can be done to the environment".

But Martin Preston, a marine pollution expert from Britain's Liverpool University, said that while there appeared to be a "cocktail" of noxious and explosive material escaping into the air and water, the impact seemed much less serious than that of the Gulf of Mexico crude oil spill two years ago: "We're not talking about anything like that," Preston said.

Methane gas in high concentrations as it emerges would risk blowing up, hence the need to clear the area, while of hydrogen sulphide he noted: "If there's a lot of it being belched out, it's horribly poisonous." However, the gas decays rapidly in air and poses little threat beyond the immediate vicinity.

The surface oil slick formed by gas condensates should, Preston said, evaporate fairly quickly, particularly in the mild, breezy conditions on the North Sea at present.

Memories are still raw in the North Sea industry of the Piper Alpha platform fire 24 years ago, when 167 people were killed in the world's deadliest offshore oil disaster.